Conventionally, in a microwave or millimeter-wave frequency band, a cross-coupled voltage controlled oscillator is used. The cross-coupled voltage controlled oscillator is a circuit which changes an oscillation frequency by controlling a voltage supplied to a control terminal. The cross-coupled voltage controlled oscillator, generally, includes a power supply terminal and a control terminal. To the power supply terminal, a fixed voltage is supplied, and to the control terminal, a control voltage for controlling the oscillation frequency is supplied.
Now, a conventional cross-coupled voltage controlled oscillator will be hereinafter described with reference to the accompanying drawings. In addition, in the following description, the “electrostatic capacitance” may be simply called the “capacitance”.
FIG. 1 is a circuit diagram illustrating one example of a configuration of the conventional cross-coupled voltage controlled oscillator.
Referring to FIG. 1, the cross-coupled voltage controlled oscillator of this conventional example includes power supply terminal 1 to which positive power supply voltage Vcc is supplied, control terminal 2 to which control voltage Vcont for controlling an oscillation frequency is supplied, output terminals 3a, 3b, cross-coupled transistors 5a, 5b whose collector terminals and base terminals are connected to each other in a cross-coupled arrangement, direct current blocking capacitances 7a, 7b for blocking a direct current, LC tanks 10a, 10b, resistor 17 for grounding emitter terminals of cross-coupled transistors 5a, 5b and grounding capacitances 21a, 21b. 
LC tank 10a, 10b is a resonant circuit including variable capacitance 8a, 8b and inductor 9a, 9b. To the collector terminals and the base terminals of cross-coupled transistors 5a, 5b, power supply voltage Vcc is supplied from common power supply terminal 1 through LC tank 10a, 10b. To the collector terminal of cross-coupled transistor 5a and the base terminal of cross-coupled transistor 5b, a common voltage is supplied from LC tank 10a, and to the collector terminal of cross-coupled transistor 5b and the base terminal of cross-coupled transistor 5a, a common voltage is supplied from LC tank 10b. 
The cross-coupled voltage controlled oscillator of this conventional example has an emitter-coupled differential amplifier formed therein by connecting the base terminals and the collector terminals of cross-coupled transistors 5a, 5b to each other in a cross-coupled arrangement. Further, the collector terminals of cross-coupled transistors 5a, 5b are connected to each other through LC tanks 10a, 10b. 
The cross-coupled voltage controlled oscillator of this conventional example is configured as described above and as a result, a positive feedback can be provided. At this time, cross-coupled transistors 5a, 5b allow electricity to travel through each other. In response to this, LC tanks 10a, 10b are excited to oscillate. With changing control voltage Vcont supplied to control terminal 2, the capacitances of variable capacitances 8a, 8b change. Accordingly, the oscillation frequency of a signal provided from output terminals 3a, 3b is controlled as shown in FIG. 2.
FIG. 2 is a graph in which control voltage Vcont supplied to control terminal 2 is shown in the horizontal axis and oscillation frequency f of a signal provided from output terminals 3a, 3b is shown in the longitudinal axis. Such a characteristic is called the “input/output characteristic of cross-coupled voltage controlled oscillator”. Further, some control voltage that is determined by a characteristic of an external circuit etc. is called the “center voltage V0”, and an oscillation frequency corresponding to center voltage V0 is called the “center frequency f0”. The input/output characteristic (voltage-frequency curve) shown in FIG. 2 shows a single band operation of the cross-coupled voltage controlled oscillator of this conventional example.
In addition, the input/output characteristic in FIG. 2 shows a characteristic of tending downward, but it may be made to tend upward. For example, when varicap diodes are used for variable capacitances 8a, 8b, the characteristic can be made to tend upward by reversing the varicap diodes and inverting the polarity of a voltage supplied to variable capacitances 8a, 8b. Also, similarly, a cross-coupled voltage controlled oscillator of the present invention can achieve both the characteristics of tending upward and downward, but description of the characteristics of tending upward and downward will be hereinafter omitted.
FIG. 3 is a circuit diagram illustrating another example of a configuration of a conventional cross-coupled voltage controlled oscillator.
Referring to FIG. 3, in the cross-coupled voltage controlled oscillator of this conventional example, resistor 17 of the cross-coupled voltage controlled oscillator shown in FIG. 1 is replaced with current source 22.
FIG. 4 is a circuit diagram illustrating still another example of a configuration of a conventional cross-coupled voltage controlled oscillator. Referring to FIG. 4, the cross-coupled voltage controlled oscillator of this conventional example includes power supply terminal 101 to which negative power supply voltage VEE is supplied, control terminal 2 to which control voltage Vcont for controlling an oscillation frequency is supplied, output terminals 3a, 3b, cross-coupled transistors 5a, 5b whose collector terminals and base terminals are connected to each other in a cross-coupled arrangement, direct current blocking capacitances 6a, 6b, 7a, 7b for blocking a direct current, LC tanks 10a, 10b, resistor 117 for connecting emitter terminals of cross-coupled transistors 5a, 5b to power supply terminal 101, and grounding resistors 33a, 33b for grounding the base terminals of cross-coupled transistors 5a, 5b. LC tank 10a, 10b is a resonant circuit including variable capacitance 8a, 8b and inductor 9a, 9b. To the emitter terminal of cross-coupled transistor 5a, 5b, negative power supply voltage VEE is supplied from power supply terminal 101 through resistor 117. To the base terminal, a voltage determined by grounding resistor 33a, 33b and a base current is supplied. With changing control voltage Vcont supplied to control terminal 2, the capacitances of variable capacitances 8a, 8b change. Accordingly, the oscillation frequency of a signal provided from output terminals 3a, 3b is controlled as shown in FIG. 2.
FIG. 5 is a circuit diagram illustrating still another example of a configuration of a conventional cross-coupled voltage controlled oscillator.
Referring to FIG. 5, the cross-coupled voltage controlled oscillator of this conventional example includes power supply terminal 101 to which negative power supply voltage VEE is supplied, control terminal 2 to which control voltage Vcont for controlling an oscillation frequency is supplied, output terminals 3a, 3b, cross-coupled transistors 5a, 5b whose collector terminals and base terminals are connected to each other in a cross-coupled arrangement, direct current blocking capacitances 6a, 6b, 7a, 7b for blocking a direct current, LC tanks 10a, 10b, resistor 117 for connecting emitter terminals of cross-coupled transistors 5a, 5b to power supply terminal 101, resistors 35a, 35b for connecting the base terminals of cross-coupled transistors 5a, 5b to power supply terminal 101, and grounding resistors 34a, 34b for grounding the base terminals of cross-coupled transistors 5a, 5b. 
LC tank 10a, 10b is a resonant circuit including variable capacitance 8a, 8b and inductor 9a, 9b. To the emitter terminal of cross-coupled transistor 5a, 5b, negative power supply voltage VEE is supplied from power supply terminal 101 through resistor 117. To the base terminal, power supply voltage VEE from power supply terminal 101 is divided by grounding resistor 34a and resistor 35a (or grounding resistor 34b and resistor 35b) that are to be supplied. With changing control voltage Vcont supplied to control terminal 2, the capacitances of variable capacitances 8a, 8b change. Accordingly, the oscillation frequency of a signal provided from output terminals 3a, 3b is controlled as shown in FIG. 2.
FIG. 6 is a circuit diagram illustrating still further example of a configuration of a conventional cross-coupled voltage controlled oscillator.
Referring to FIG. 6, the cross-coupled voltage controlled oscillator of this conventional example includes power supply terminal 101 to which negative power supply voltage VEE is supplied, control terminal 2 to which control voltage Vcont for controlling an oscillation frequency is supplied, output terminals 3a, 3b, cross-coupled transistors 5a, 5b whose collector terminals and base terminals are connected to each other in a cross-coupled arrangement, direct current blocking capacitances 6a, 6b, 7a, 7b for blocking a direct current, LC tanks 10a, 10b, resistor 117 for connecting emitter terminals of cross-coupled transistors 5a, 5b to power supply terminal 101, grounding resistors 33a, 33b for grounding the base terminals of cross-coupled transistors 5a, 5b and capacitance bank 36.
LC tank 10a, 10b is a resonant circuit including variable capacitance 8a, 8b and inductor 9a, 9b. Capacity bank 36 includes a plurality (n) of capacitances 37a, 37b and a plurality of switches 38. To the emitter terminal of cross-coupled transistor 5a, 5b, negative power supply voltage VEE is supplied from power supply terminal 101 through resistor 117. To the base terminal, a voltage determined by grounding resistor 33a, 33b and a base current is supplied. With changing control voltage Vcont supplied to control terminal 2, the capacitances of variable capacitances 8a, 8b change. Accordingly, the oscillation frequency of a signal provided from output terminals 3a, 3b is controlled as shown in FIG. 2. Further, depending on the combination of on/off states of the plurality of switches 38 of capacitance bank 36, center frequency f0 is discretely controlled. As the result, a plurality of characteristics discretely controlled as shown in FIG. 7 can be achieved.
FIG. 7 is a graph in which control voltage Vcont supplied to control terminal 2 is shown in the horizontal axis and oscillation frequency f of a signal provided from output terminals 3a, 3b is shown in the longitudinal axis. The input/output characteristic shown in FIG. 7 shows that the cross-coupled voltage controlled oscillator of the conventional example performs a multi-band operation. In addition, as for the related art for discretely controlling the input/output characteristic, there are technologies disclosed in Japanese Patent Laid-Open No. 2004-120215 and No. 2004-159222.
FIG. 8 is a circuit diagram illustrating still another example of a configuration of a conventional cross-coupled voltage controlled oscillator.
Referring to FIG. 8, the cross-coupled voltage controlled oscillator of this conventional example includes power supply terminal 101 to which negative power supply voltage VEE is supplied, control terminal 2 to which control voltage Vcont for controlling an oscillation frequency is supplied, output terminals 3a, 3b, cross-coupled transistors 5a, 5b whose collector terminals and base terminals are connected to each other in a cross-coupled arrangement, direct current blocking capacitances 6a, 6b, 7a, 7b for blocking a direct current, LC tanks 110a, 110b, resistor 117 for connecting emitter terminals of cross-coupled transistors 5a, 5b to power supply terminal 101 and grounding resistors 33a, 33b for grounding the base terminals of cross-coupled transistors 5a, 5b. 
LC tank 110a, 110b is a resonant circuit including variable capacitance 8a, 8b and variable inductor 39a, 39b. To the emitter terminal of cross-coupled transistor 5a, 5b, negative power supply voltage VEE is supplied from power supply terminal 101 through resistor 117. To the base terminal, a voltage determined by grounding resistor 33a, 33b and a base current is supplied. With changing control voltage Vcont supplied to control terminal 2, the capacitances of variable capacitances 8a, 8b change. Accordingly, the oscillation frequency of a signal provided from output terminals 3a, 3b is controlled as shown in FIG. 2. Further, by controlling the inductance value of variable inductor 39a, 39b, center frequency f0 is continuously controlled. As the result, a plurality of input/output characteristics continuously controlled as shown in FIG. 9 can be achieved.
FIG. 9 is a graph in which control voltage Vcont supplied to control terminal 2 is shown in the horizontal axis and oscillation frequency f of a signal provided from output terminals 3a, 3b is shown in the longitudinal axis. The input/output characteristic shown in FIG. 9 shows that the cross-coupled voltage controlled oscillator of the conventional example performs a continuous multi-band operation.
As described above, in the cross-coupled voltage controlled oscillators shown in FIGS. 1, 3, 4 and 5, only one input/output characteristic as shown in FIG. 2 can be achieved. That is, these cross-coupled voltage controlled oscillators can not perform a multi-band operation, because they perform a single band operation. When these cross-coupled voltage controlled oscillators are used in a phase-locked loop (PLL) circuit employed for radio communication systems etc, their operation in a plurality of frequency bands (multi-band) can not be carried out. Further, when these cross-coupled voltage controlled oscillators are used in a clock and data recovery (CDR) circuit etc. employed for optical communication systems etc, their operation at a plurality of bit rates can not be carried out.
Further, in the cross-coupled voltage controlled oscillator shown in FIG. 6, because a plurality of discrete input/output characteristics can be achieved, a multi-band operation can be performed. However, because a capacitance bank including a plurality of capacitances and a plurality of switches is necessary for this cross-coupled voltage controlled oscillator, the circuit becomes complex. Therefore, it is difficult to provide a multi-band operation especially in a very high frequency band such as a millimeter-wave frequency band by using this cross-coupled voltage controlled oscillator.
Further, because even the cross-coupled voltage controlled oscillator shown in FIG. 8 can provide a plurality of continuous input/output characteristics, a multi-band operation can be achieved. However, because the variable inductor is usually formed by technologies such as MEMS (Micro Electro Mechanical Systems), this cross-coupled voltage controlled oscillator becomes complex in structure and in the production process. Therefore, this cross-coupled voltage controlled oscillator is unsuitable, especially in a very high frequency band such as a millimeter-wave frequency band.